Shorten video with speed control effect strip already applied
The UI of the Speed Strips are super unintuitive. I've tried to address the subject matter here without any success: https://developer.blender.org/D6110
There are four ways to change the speed of the strip, using the speed strip options. I hope listing them here will help you and others out:
&quot;Stretch to input strip length&quot;

The &quot;Use as Speed&quot; does exactly the same as the Multiply Speed:

If &quot;Scale to Length&quot; is selected the strip duration goes from 0%(first frame) to 100%(last frame) and those values needs to be animated:

When &quot;Scale to Length&quot; is unselected, and behaves like the previous setting, but in frame numbers.